c++ - 为什么我需要在以下示例中的 block_cache.h 之前包含 block_cache_key.h 和 block.h？

Question

通常我们会使用标准类型作为 std::unordered_map<key, value> 的键和值.但现在我需要自定义我自己的键和值类。

键类在block_cache_key.h 中定义如下:

#ifndef BLOCK_CACHE_KEY_H_
#define BLOCK_CACHE_KEY_H_

#include <functional>

namespace wcg{
class BlockCacheKey{
public:
    BlockCacheKey(const std::string &name, int64_t offset) : name_(name), offset_(offset) {}

    bool operator==(const BlockCacheKey &other) const{
        return offset_ == other.offset_ && name_ == other.name_;
    }

    const std::string &name() const{
        return name_;
    }

    const int64_t offset() const{
        return offset_;
    }

    std::string to_string() const{
        return name_ + "_" + std::to_string(offset_);
    }

private:
    std::string name_;
    int64_t offset_;
};
}

namespace std{
    template <>
    class hash<wcg::BlockCacheKey>{
    public:
        size_t operator()(const wcg::BlockCacheKey &key) const{
            return std::hash<std::string>()(key.name()) ^ (std::hash<int64_t>()(key.offset()));
        }
    };
}

#endif

值类在 block.h 中定义如下:

#ifndef BLOCK_H_
#define BLOCK_H_

namespace wcg{
class Block{
public:
    Block() {}
};
}

#endif

以及使用 std::unorded_map 的类在 block_cache.h 中定义如下:

#ifndef BLOCK_CACHE_H_
#define BLOCK_CACHE_H_

#include <iostream>
#include <memory>
#include <unordered_map>

namespace wcg{

class BlockCacheKey;
class Block;

class BlockCache{
public:
    BlockCache() : count_(0) {
        std::cout << "initial stats: " << to_string() << std::endl;
    }

    ~BlockCache(){
        std::cout << "final stats: " << to_string() << std::endl;
    }

    void CacheBlock(const BlockCacheKey &key, std::shared_ptr<Block> block){
        std::cout << "cache put: " << key.to_string() << std::endl;
        map_[key] = block;
        count_++;
    }

    std::shared_ptr<Block> GetBlock(const BlockCacheKey &key){
        auto pos = map_.find(key);
        if(pos != map_.end()){
            std::cout << "cache get: " << key.to_string() << std::endl;
            return pos->second;
        }
        return nullptr;
    }

    std::string to_string() const{
        return "block count: " + std::to_string(count_);
    }

private:
    std::unordered_map<BlockCacheKey, std::shared_ptr<Block>> map_;
    int count_;
};
}

#endif

主要功能定义在main.cpp中如下:

#include <memory>

#include "block_cache_key.h"
#include "block.h"
#include "block_cache.h"

// g++ -std=c++11 -g -Wall main.cpp

int main(int argc, char* argv[]){
    wcg::BlockCache bc;
    return 0;
}

当 #include "block_cache.h"终于，编译OK了。但是当#include "block_cache.h"在 #include "block_cache_key.h" 之前和 #include "block.h" ，编译会失败，错误信息非常难看，很难理解。

我已经知道的，std::unordered_map是一个模板。GCC在编译时，会先对include部分进行预处理，即全部展开。这给我的印象是包含顺序不是问题。

最后要注意的是，我不想包括 block_cache_key.h和 block.h在 block_cache.h

部分编译错误信息:(完整的错误消息在 https://paste.ubuntu.com/p/VyrWKCTG4q/ 中)

In file included from /usr/include/c++/7/bits/hashtable.h:35:0,
                 from /usr/include/c++/7/unordered_map:47,
                 from block_cache.h:13,
                 from main.cpp:5:
/usr/include/c++/7/bits/hashtable_policy.h: In instantiation of 'struct std::__detail::__is_noexcept_hash<wcg::BlockCacheKey, std::hash<wcg::BlockCacheKey> >':
/usr/include/c++/7/type_traits:143:12:   required from 'struct std::__and_<std::__is_fast_hash<std::hash<wcg::BlockCacheKey> >, std::__detail::__is_noexcept_hash<wcg::BlockCacheKey, std::hash<wcg::BlockCacheKey> > >'
/usr/include/c++/7/type_traits:154:31:   required from 'struct std::__not_<std::__and_<std::__is_fast_hash<std::hash<wcg::BlockCacheKey> >, std::__detail::__is_noexcept_hash<wcg::BlockCacheKey, std::hash<wcg::BlockCacheKey> > > >'
/usr/include/c++/7/bits/unordered_map.h:103:66:   required from 'class std::unordered_map<wcg::BlockCacheKey, std::shared_ptr<wcg::Block> >'
block_cache.h:50:60:   required from here
/usr/include/c++/7/bits/hashtable_policy.h:87:34: error: no match for call to '(const std::hash<wcg::BlockCacheKey>) (const wcg::BlockCacheKey&)'
  noexcept(declval<const _Hash&>()(declval<const _Key&>()))>
           ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/7/bits/move.h:54:0,
                 from /usr/include/c++/7/bits/stl_pair.h:59,
                 from /usr/include/c++/7/bits/stl_algobase.h:64,
                 from /usr/include/c++/7/memory:62,
                 from main.cpp:3:
/usr/include/c++/7/type_traits: In instantiation of 'struct std::__not_<std::__and_<std::__is_fast_hash<std::hash<wcg::BlockCacheKey> >, std::__detail::__is_noexcept_hash<wcg::BlockCacheKey, std::hash<wcg::BlockCacheKey> > > >':
/usr/include/c++/7/bits/unordered_map.h:103:66:   required from 'class std::unordered_map<wcg::BlockCacheKey, std::shared_ptr<wcg::Block> >'
block_cache.h:50:60:   required from here
/usr/include/c++/7/type_traits:154:31: error: 'value' is not a member of 'std::__and_<std::__is_fast_hash<std::hash<wcg::BlockCacheKey> >, std::__detail::__is_noexcept_hash<wcg::BlockCacheKey, std::hash<wcg::BlockCacheKey> > >'
     : public __bool_constant<!bool(_Pp::value)>
                               ^~~~~~~~~~~~~~~~
In file included from /usr/include/c++/7/unordered_map:48:0,
                 from block_cache.h:13,
                 from main.cpp:5:
/usr/include/c++/7/bits/unordered_map.h: In instantiation of 'class std::unordered_map<wcg::BlockCacheKey, std::shared_ptr<wcg::Block> >':
block_cache.h:50:60:   required from here
/usr/include/c++/7/bits/unordered_map.h:103:66: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<std::hash<wcg::BlockCacheKey> >, std::__detail::__is_noexcept_hash<wcg::BlockCacheKey, std::hash<wcg::BlockCacheKey> > > >'
       typedef __umap_hashtable<_Key, _Tp, _Hash, _Pred, _Alloc>  _Hashtable;
                                                                  ^~~~~~~~~~
/usr/include/c++/7/bits/unordered_map.h:110:45: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<std::hash<wcg::BlockCacheKey> >, std::__detail::__is_noexcept_hash<wcg::BlockCacheKey, std::hash<wcg::BlockCacheKey> > > >'
       typedef typename _Hashtable::key_type key_type;

Answer 1

让我们看看BlockCache.h .你向编译器保证 Block和 BlockCacheKey存在并最终将被定义——在这一点上它们是不完整的类型。只要您不以要求它们完整的方式使用它们，就可以正常工作:形成引用、指针甚至 std::shared_ptr s 到不完整的类型是可以的。参见 here获取您不能用它们做的事情的列表。

您使用 BlockCacheKey 违反了此规定(和 Block )在 std::unordered_map<BlockCacheKey, std::shared_ptr<Block>> 中- 实例化 std::unordered_map<Key, Value>模板(声明该类型的成员所必需的)，Key和 Value类型必须完整。

想象一下 sizeof(std::unordered_map<Key, Value>)取决于 sizeof(Key) (这将在其权利范围内)。只有前向声明 Key , sizeof(Key)将是未知的，因此 sizeof(std::unordered_map<Key, Value>)将是未知的，因此你的 BlockCache 的大小将是未知的(即使在您定义它之后!)。编译器无法处理它，这就是为什么不允许您这样做的原因。

Last thing to note, I do not want to include block_cache_key.h and block.h in block_cache.h

这是不可能的(正如您所观察到的那样，不需要用户发出时髦的包含命令)。实例化 std::map<BlockCacheKey, Whatever> , BlockCacheKey 的定义必须知道，你只能从包含它的标题中得到。我相信 Block 也是如此在std::shared_ptr<Block>您打算用作 map 值类型的类型。